Temperature control for exothermic reactions has been achieved in the past in a number of ways. One such method has been to place cooling coils within the catalyst bed to remove the heat of reaction. This type of reactor has commonly been referred to as an adiabatic reactor. While this arrangement is useful on a small scale, larger reactors and catalyst beds present problems. The low liquid velocities required to achieve the desired residence times in the catalyst bed provide very poor heat transfer coefficients and inefficient cooling.
Another method used, especially where one of the reactants is gaseous, is to introduce cold reactant or "quenches" along the length of the catalyst bed. This type of cooling has been effectively used in such reactors as hydrocrackers where large quantities of hydrogen are needed. The amount of quench used is limited by compressors and other related gas recycle equipment. Additionally, distribution of the cold gas in larger catalyst beds can create such problems as hot spots.
The adiabatic and quench reactors noted above remove only sensible heat in the reactors and are thus limited by the specific heats of the reactants or quench gases. More efficient temperature control can be achieved in a boiling point or isothermal reactor where the reaction is carried out at a pressure so as to cause vaporization of a portion of the reaction mixture. The latent heat of vaporization absorbs the exothermic heat of reaction and limits the temperature rise in the bed. This, too, has limitations in that in highly exothermic reactions all of the reaction mixture may be vaporized before the desired conversion of reactants is achieved. To solve this problem several reactors may be used in series with the effluent from one reactor being condensed before introduction to the next reactor. In this type of reactor the entire reaction mixture passes through and out of the reactor and contains reactants, products, liquids and gases.
A given composition, the reaction mixture, will have a different boiling point at different pressures, hence the temperature in the reactor is controlled by adjusting the pressure to the desired temperature within the recited range. The boiling point of reaction mixture thus is the temperature of the reaction and the exothermic heat of reaction is dissipated by vaporization of the reaction mixture. The maximum temperature of any heated liquid composition will be the boiling point of the composition at a given pressure, with additional heat merely causing more boil up. The same principal operates in the present invention to control the temperature. There must be liquid present, however, to provide the boil up, otherwise the temperature in the reactor will continue to rise until the catalyst is damaged. This is a substantial departure from the prior art for this type of reactor, where sufficient pressure was employed to maintain the reaction mixture in liquid phase.
The present invention which relates to the liquid phase type of reaction also provides means for removing heat from the fixed continuous catalyst bed. These and other advantages will become apparent from the following descriptions.